Divers go deeper to uncover the impact of climate change on coral reefs

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A range of cutting-edge technologies are allowing divers to go deeper into the ocean than ever before, enabling new research into the health of coral reefs and how to help these vital ecosystems cope with climate change.

Climate change has made coral reefs one of the earth's most-threatened ecosystems. Rising ocean temperatures and increasing acidity, due to carbon dioxide dissolving in the water, cause the symbiotic algae that live inside a coral’s tissue - and are its primary food source - to leave. The algae also give corals their color so when they leave, the coral turns white, a devastating event called bleaching. Around half of the Great Barrier Reef's corals were killed by bleaching in 2016 and 2017, for example.

Coral reefs are a vital part of marine life. They also provide flood protection and play an important role in the fishing and tourism industries. In the hope of protecting them from further damage, scientists are keen to learn more.

Of particular interest are mesophotic corals, which grow between 30 and 150 meters below the ocean surface, where temperatures are lower and there is less light penetration. One hypothesis is that this is an area in which corals can find refuge from the effects of climate change. That has been hard to test until now because divers could not easily get further than 60 meters below the surface to inspect these corals. However, thanks to new technologies, they now can.

Diving into the Twilight Zone

Since 2010, an expedition group called Under The Pole (UTP) has been exploring the world's oceans and pushing the boundaries of diving. Previous missions have taken them to the Arctic and Greenland. The team is currently on a three-year global expedition, based for now in French Polynesia, where they are exploring the ocean’s Twilight Zone – up to 150 meters below the surface.

The team can reach those depths using rebreathers, which absorb the carbon dioxide a diver exhales, recycle the unused oxygen and add gas to replace the consumed oxygen, allowing them to dive deeper and stay underwater longer. Down there are species of fish that we know little about and the mesophotic coral reef that is of great interest to scientists. But it's also dark, cold and dangerous.

“You have to be an experienced diver to use rebreathers,” says Erwan Marivint of UTP. “You must carefully manage the balance of gases and dive with an alternative breathing source if the rebreather fails. But the rebreathers let us stay underwater from three to six or seven hours. We can reach corals that no human has ever reached and take samples that will help us understand them.”

Deep learning technology

The team’s researchers want to know why these corals live in such deep water, what survival tricks they use to flourish in the Twilight Zone, and what they can teach us about corals on the surface. Are they moving deeper to escape the effects of global warming? Or are they entirely different from their shallower-water relatives?

“We’ve been looking at zooxanthellate, or light-dependent, corals,” says Laetitia Hedouin, the scientific director of the coral study from the CRIOBE. “One zooxanthellate coral species, Leptoseris hawaiiensis, broke a world depth record at 172m. This raises questions regarding how these corals adapt to live in low-light conditions. The answer may help us to replenish shallow reefs that have been damaged by climate change."

The latest stage of UTP's exploration uses a pressurized aluminum capsule that allows a team of three divers to stay underwater for up to 72 hours. This four cubic-meter underwater habitat offers little room to relax and it’s not exactly comfortable: humidity inside is 90 percent.

However, it allows divers to rest and feed without returning to the surface every six hours. They only leave the capsule twice a day to take samples from coral reefs or fish.

Powering exploration

None of this would be possible without power, of course. The rebreathers are powered by Saft LS14500 primary lithium cells, while the capsule uses Saft lithium-ion (Li-ion) cells for on-board electronics systems. Even the schooner from which operations are managed, the WHY, relies on eight Xcelion® 6T Li-ion batteries to power the scientific and diving equipment on board.

“The key for batteries for this kind of mission is safety and reliability,” says Olivier Goujon of Saft. “The batteries also need to work perfectly in a wide range of temperatures, whether the team are in a very warm environment like French Polynesia or a very cold one like the Antarctic.”

Marivint says the team has already discovered new species of coral, though further revelations will have to wait until UTP's scientists are ready to publish their findings. “Very little was known about these corals before our work,” he adds. “This is now the most important collection of data on mesophotic corals, anywhere in the world.”

The hope is that this research will help halt the destruction of the world's coral reefs, perhaps even point the way to restoring them to health – and shed light on some of the darkest parts of the ocean.